Growth hormone (GH) and insulin-like growth factor-I (IGF-I) have been shown to be important for fertility, especially in GH-deficient individual that display a lower fertility rate. Growth hormone deficiency or insufficiency can cause a delay in the onset of puberty, unless treated with synthetic GH. It is thought that GH affects the ovary during puberty both indirectly through the gonadotropins and IGF-I and directly through its effect on steroidogenesis. The GH axis is activated by small increases in circulating estrogens, which initiate large increases in GH during puberty. The reproductive function of the female is also affected by GH. GH acts on the ovary affecting gametogenesis and steroidogenesis. GH receptor mRNA and protein have been found in the ovarian cell, which suggests that direct action of GH provides an important modulatory effect on gonadotropin's dependent and independent functions. It also affects the maturation of the follicle and gamete, and thereby plays a facilitatory role in fertility. The majority of women with GH-deficiency, but not all, require assisted reproductive technologies to induce ovulation.
Overexpression of GH also negatively impacts reproduction. Reproduction life span and efficiency are reduced in both sexes, with the severity and frequency of reproductive deficits being related to plasma bGH levels. Most transgenic females expressing high levels of bGH are sterile due to luteal failure. In mouse, overexpression of human GH, which interacts with both GH and PRL receptors, leads to additional endocrine and reproductive abnormalities including stimulation of LH beta mRNA levels and LH secretion, loss of responsiveness to testosterone feedback, overstimulation of mammary glands, enhanced mammary tumorigenesis, and hypertrophy of accessory reproductive glands in males.
In a model of GHRH-KO (Alba, Endocrinology. 2004 September; 145(9):4134-43) male homozygous animals had normal copulatory behavior and fertility. When mated with heterozygous females, no differences in terms of litter size (8.3 pups/litter) or Mendelian ratio for offspring was observed. On the contrary, homozygous females, although maintaining normal fertility, had a consistent reduction in litter size (average, 4.1 pups/litter). All homozygous adult females displayed normal duration of gestation (19-21 d), normal pup retrieval, and normal maternal behavior.
Hormone therapies with protein or peptide hormones, agonists, and antagonists are short-lived in vivo and have required frequent injections or depot delivery to elicit long-term effects on physiologic systems. In many instances, protein hormone therapy can be inefficient and labor intensive. This is due in part to the cost, availability, and pharmacokinetics of many protein preparation.
Recently, the ability to transfect DNA into adult mammals has overcome the barriers of impracticality and economic infeasibility associated with long-term protein hormone therapy. Plasmid therapy has evolved over the past decade into a safe approach for delivery of DNA and their gene product in vivo (Prud'homme, Curr Gene Ther. 2006 April; 6(2):243-73). Combining new plasmid delivery technologies with the elucidation of genetic information for domestic mammals could expand the use of this technique in future therapies. Delivery of plasmids by direct intramuscular injection followed by a physical method to enhance plasmid uptake and expression, such as electroporation, has shown to be successful in several species of animals. This approach has been applied in autoimmune and/or inflammatory diseases, DNA vaccination against infectious agents (e.g., hepatitis B virus, human immunodeficiency virus-1) or tumor antigens (e.g., HER-2/neu, carcinoembryonic antigen) (Curcio, Cancer Gene Ther. 2008 February; 15(2):108-14) (Hirao, Vaccine. 2008 Jan. 17; 26(3):440-8).
In various species, GH treatment has been beneficial on numerous physiologic systems. In the normal horse, GH has been evaluated for its effects on the cardiovascular system, the musculoskeletal and immune system, and the reproductive axis. Growth hormone treatment has resulted in increased granulocyte number and musculation in aged mares, increased number of small follicles on the ovaries, and increased accessory sex gland function in stallions. In the horse, plasma GH concentrations can be increased by various secretagogues, feeding, and exercise. Pharmacological doses of GHRH are known to increase GH.
Researchers have yet to examine the effects of chronic GH treatment on many physiologic systems in the horse. Because of its short half-life, the use of GHRH to physiologically stimulate the GH axis was not investigated. Nevertheless, previous work showed that by using optimized plasmid constructs, this technique can be applied to impact long-term hormonal and clinical parameters in normal and pathologic circumstances in large mammals, such as dogs, cows, and pigs, using a low plasmid quantity with the absence of adverse effects (reviewed in Draghia-Akli, Comb Chem High Throughput Screen. 2006 March; 9(3):181-5).
Treatment with gonadotropin releasing hormone (GnRH), the hypothalamic peptide regulating pituitary LH and FSH secretion, and its analogues have shown promise for regulating reproductive traits.
GnRH therapy has shown promise as a treatment for improving gonadotropin and testosterone secretion in these scenarios. Chronic, pulsatile administration of GnRH increases LH and testosterone secretion but is less effective on semen motility in stallions. Pulsatile therapy is, however, impractical due to the labor needed and the long-term nature of the treatment. Long-lasting, potent analogues of GnRH initially increase gonadotropin concentrations but subsequently down-regulate gonadotropin production.
There remains a need for compositions and methods of enhancing fertility in mammals. Also needed are methods of enhancing fertility in normal and subfertile mammals, including such methods that also provide economic benefit, which would allow for prophylactic treatments.